We study a group of inborn errors of metabolism, the hereditary methylmalonic acidemias (MMA) and disorders of intracellular cobalamin metabolism, in the clinic and laboratory. Affected patients are medically fragile and suffer from multisystemic complications such as severe metabolic instability, stroke of the basal ganglia, pancreatitis, end stage renal failure, growth impairment, osteoporosis, and developmental delay. Despite intensive study since the early 1960s, few patients with MMA have survived into adulthood and evidence for effectiveness of current medical therapies is lacking, a fact that stands in stark contrast to the practice of screening all US newborns for MMA. Because these disorders feature pathology that can be seen in many common conditions, such as vitamin B12 deficiency, stroke syndromes, pancreatic dysfunction, diabetes, chronic kidney disease, osteoporosis, and obesity, it is likely that the careful elucidation of patient phenotypes will provide new insights into the pathophysiology of more complex and prevalent disorders, and perhaps, suggest new pathways to target for therapeutic intervention that could affect millions of patients. The translational research program has three major interrelated goals. Goal 1 is to define the natural history of the conditions using clinical research; Goal 2 is to investigate the disorders in the laboratory using metabolic, genetic and genomic approaches and Goal 3 is to develop new treatments for the disorders, focusing gene therapy strategies. The clinical characterization of patients with methylmalonic acidemia (MMA) and related disorders, via a dedicated natural history study, NHGRI protocol Clinical and Basic Investigations of Methylmalonic Acidemia and Related Disorders (ClinicalTrials.gov Identifier: NCT00078078) has continued. Through our clinical protocol, we have continued to accrue patients with MMA and cobalamin metabolic disorders and have evaluated > 170 affected individuals; this is the largest single center cohort of such patients in the world. We have continued to focus on the clinical characterization of patients to expand our understanding of the natural history of MMA, and have published articles on the stroke syndrome of MMA (reference 1), the rehabilitation medicine phenotype of MMA (reference 2), the use of anesthesia in MMA (reference 3), and pregnancy in MMA (reference 4). All these manuscripts used clinical data derived from the study of patients in the NIH Clinical Center, admitted and assessed through our natural history protocol. One (reference 4) has spawned a new protocol in the section focused on defining the effects of inborn errors of metabolism on pregnancy (Maternal Inborn Errors of Metabolism in Pregnancy: A Pregnancy Registry Protocol ClinicalTrials.gov Identifier: NCT02322177). Active efforts include a description of the state of MMA post solid organ transplantation, delineation of the natural history and ophthalmological phenotypes in patients with MUT MMA and cblC deficiency, and defining the metabolic phenotype of MMA patients with stable isotopes. Another area of active clinical research effort has broadly focused on therapies in MMA. We provided a commentary on the use of solid organ transplantation to treat MMA patients (reference 5) and very recently published two papers that critically assess the use of medical foods to treat patients with isolated MMA (reference 6) or a related disorder caused by impaired cobalamin metabolism, cblC deficiency (reference 7). The latter two papers will lead to the development of new formulas for patients with MMA and hopefully, a joint IRP /extramural clinical trial. Lastly, a paper from an older collaboration has also appeared ( reference 8). Laboratory investigations have continued to focus on generating and characterizing animal models of MMA and cobalamin disorders as well as pre-clinical gene therapy studies. We have used zebrafish to model methylmalonyl-CoA mutase (MUT) MMA and cobalamin C deficiency and expect to publish these models in the next year. We have also used mice to create knock-out models of MMAA deficiency, which causes a relatively common and severe form of vitamin B12 responsive MMA, and combined malonic-methylmalonic acidemia (CMAMMA) due to acyl-coA synthase family member 3 (ACSF3) deficiency. Finally, we have used genome editing to create the first viable mouse model of cobalamin C deficiency. In the next year, we will characterize the mutant animals using genomic, proteomic and metabolomic approaches to define mechanisms and identify biomarkers that can be translated to patient care, and test new therapeutics. We have continued to study viral gene therapy as treatment for methylmalonyl-CoA mutase (MUT) deficiency. During the past cycle, and in collaboration with Dr Shawn Burgess of the NHGRI IRP, we have defined the genotoxicity of adeno-associated viral AAV gene therapy in mice. Our publication in the Journal of Clinical Investigation (reference 9) described the results from almost 5 years of preclinical gene therapy studies in mice with MMA and controls. We have convincingly demonstrated an association between AAV treatment and the development of hepatocellular carcinoma (HCC) and using genomics, have defined the mechanism by which this occurs. By comparing the integration profiles from varied AAV vectors, we discovered that the enhancer-promoter carried by the vector seems to determine the risk for the development of an HCC after exposure to AAV. We also developed and tested an AAV vector that did not cause HCC in the mice and are moving forward with efficacy and dose-finding studies using this vector and others under development. A patent has been filed on modified MUT transgenes and their use in gene therapy (reference 10). We plan to use our studies to enable an IND submission to the FDA as a step toward translating gene therapy to the clinic. We have also continued to serve as a resource for other groups in the NHGRI, and have successfully generated a new AAV vector to treat Niemann-Pick Type C (NPC) disease. In collaboration with Dr William Pavans group in the NHGRI IRP, we have tested this vector in Npc mice and filed a patent (reference 11), setting the stage for the development of gene therapy for this lethal neurometabolic disorder within the NIH IRP.